SPN 3250 FMI 20 – DOC Temperature Drift - Inlet Low or Outlet High Fault Code (Symptoms, Causes & Fix)

Sep 13, 2026

The SPN 3250 FMI 20 fault code indicates a temperature drift issue within the Diesel Oxidation Catalyst (DOC) system, specifically when the inlet temperature is too low or the outlet temperature is too high. This diesel engine fault code is triggered when the Engine Control Module (ECM) detects a discrepancy in the expected temperature readings from the DOC temperature sensors, suggesting potential sensor faults or issues within the exhaust aftertreatment system.

What SPN 3250 FMI 20 Means

The SPN 3250 FMI 20 diagnostic trouble code is associated with the Diesel Oxidation Catalyst (DOC) temperature sensors. It signifies that the ECM has identified a drift in temperature readings, either at the inlet being too low or the outlet being too high. This could be due to a faulty sensor or issues affecting the sensor's ability to accurately report temperatures, such as wiring problems or exhaust leaks.

Common Causes of This Fault Code

Faulty DOC temperature sensor
Wiring harness damage or corrosion
Exhaust leaks before or after the DOC
DOC substrate damage or clogging
ECM signal processing issues

Symptoms Drivers May Notice

When the SPN 3250 FMI 20 fault code is active, drivers may notice a check engine light or a warning indicator on the dashboard. There might be a reduction in engine performance, and in some cases, the vehicle may enter a derate mode to protect the engine and aftertreatment system. Additionally, there could be increased exhaust emissions or unusual exhaust smells.

How Technicians Diagnose This Fault

Technicians begin diagnosing the SPN 3250 FMI 20 fault by performing a visual inspection of the DOC and its associated components. This includes checking the wiring harness for any signs of damage or corrosion. Next, they verify the power supply and ground circuits to the temperature sensors using a multimeter. Voltage reference checks are also conducted to ensure proper sensor operation. Using diagnostic software, technicians will read live data to confirm the temperature readings and compare them against expected values. If necessary, a smoke test may be performed to detect any exhaust leaks that could affect sensor readings.

Common Misdiagnosis

A common misdiagnosis with the SPN 3250 FMI 20 fault code is assuming a faulty DOC without thoroughly verifying the integrity of the sensors and wiring. Technicians may also overlook minor exhaust leaks that can significantly impact temperature readings, leading to unnecessary sensor replacements.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 3250 FMI 20 fault code active, it is not advisable. The fault can lead to engine derate or protection mode, reducing performance and potentially causing further damage to the aftertreatment system. It is recommended to address the issue promptly to avoid more severe consequences.

Mechanic's Diagnostic Tip

When diagnosing the SPN 3250 FMI 20 fault code, always verify the sensor's power and ground circuits before replacing any components. A simple continuity check can save time and prevent unnecessary part replacements. Additionally, using a thermal imaging camera can help identify temperature inconsistencies across the DOC, providing a clearer picture of potential issues.

Frequently Asked Questions

What does SPN 3250 FMI 20 indicate? It indicates a temperature drift issue in the DOC system, with potential sensor or exhaust system faults.
Can a faulty sensor cause SPN 3250 FMI 20? Yes, a faulty DOC temperature sensor is a common cause of this fault code.
Is it safe to drive with SPN 3250 FMI 20 active? Driving is possible but not recommended, as it may lead to engine derate or further damage.